require setuptools and do a legitimate "python setup.py test"

[Mike Bayer]

pytest is pretty much set on this way for "python setup.py test" to work without silliness, so let's just drop the "run without setuptools" thing, a setuptools-less Python environment these days does not exist.

this is important because the hack we do right now at the end says "ran one test", and looks ridiculous.

[Mike Bayer]

• remove "distutils-only" mode, use setuptools only
• add first-class "pytest" runner to setup.py, fixes #3489
• remove references to setuptools Feature and mark that "--without-cextensions" is removed, fixes #3500

→ <<cset 5ab9ab7940a6>>